People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

The question of altruism has troubled researchers for a long time, particularly the idea of standing up to someone. Why would someone choose to stand up to another person, who might be larger or stronger, in order to defend a weaker person – particularly if it could have disastrous consequences? What's more, why is this something that occurs not just in humans, but in animals across the board?

The question puzzled Charles Darwin for years. Now, one mathematician from the University of Tennessee believes that he has devised an answer: fighting the power is better for society, evolutionarily.

Sergey Gavritles, who is a distinguished professor of ecology, evolutionary biology, and mathematics, studied egalitarian syndrome and society – particularly reciprocity, kinship, group selection, and punishment – and extrapolated reasons that those various phenomena exist, by creating an evolutionary model for individuals who live in a group, and who are competing for the same resources. His results, published in the Proceedings of the National Academy of Sciences, may be surprising.

In his abstract, he says, "First, I show that the logic of within-group competition implies under rather general conditions that each individual benefits if the transfer of the resource from a weaker group member to a stronger one is prevented. This effect is especially strong in small groups."

From there, he says that a "genetically controlled" psychology takes hold in group members that causes people to intervene in a conflict between a bully and a victim, on the side of the victim. It is better for the group if the majority of people are sided against the bully, and egalitarianism is better for everyone because it reduces inequality within the group. He says that, from this psychology, "inequity aversion, empathy, compassion, and egalitarian moral values via the internalization of behavioral rules" evolved.

Gavritles divides the group into two classifications of people: stronger members, who can receive the best resources; and helpers, who aid the weaker members. In societies with more helpers, the group as a whole was more prosperous.

By avoiding the monopolization of resources, everyone in the group could thrive and have more offspring, perpetuating the underdog psychology through natural selection.